Detection and tracking of submarines has long been the goal of antisubmarine warfare. In the past, sonobuoys and the like have been used to sense the presence of submerged submarines through their deployment and the use of sophisticated sonar techniques. Additionally, worldwide hydrophone arrays have been utilized in an attempt to locate and track submarines.
However, with passive acoustic techniques, the development of noiseless or silent submarines oftentimes makes it impossible to detect such quiet submarines due to their quiet-running design. Moreover, installing or deploying hydrophone arrays to detect the presence of submarines is both costly and time-consuming, thus significantly limiting the search area.
With respect to magnetometers, in order to detect a magnetic anomaly, one must first have a magnetic map of the terrain against which to compare the magnetometer readings from an overlying aircraft. The development of these maps with sufficient accuracy has been problematic in the past and the isolation of the magnetometer from the aircraft itself, usually on a boom, presents it own problems.
As described in U.S. Pat. No. 4,893,294, a different system for detecting the presence of submarines by Donald A. Leonard and Harold E. Sweeney involves the sensing of thermoclines which are produced by sub-sea wake turbulence when a submarine moves through the water. As described in the Encyclopedia of Oceanography, Vol. 1, Rhodes Pairbridge Edition, pp. 402-408, Rhineholt Publishing Corporation of New York, 1996, subsurface waves are produced by the movement of a body through the water which creates so-called internal waves. The above patent describes a way to measure the internal waves by constructing a profile of ocean water temperatures as a function of depth. While techniques have been employed in the past for measuring these profiles involving the use of many temperature sensors, these are costly and time-consuming to deploy, making them unsuited to high spatial and temporal resolution coverage of large ocean areas.
In an effort to improve on the early techniques for measuring the thermoclines, thus to detect submarines by measurement of sub-surface ocean temperatures, the above-noted patent describes a system for remote temperature sensing by using a laser carried by an attack or search submarine. In this patent the remote temperature sensing device uses a pulsed laser to illuminate an area of interest which includes the thermocline region. Part of the temperature sensing device is an optical receiver which picks up the laser backscatter and uses the self-heterodyne of the wavelength-shifted Brillouin scatter with the unshifted Rayleigh scatter.
According to this patent the self-heterodyne action allows mixing of the above signals from each volume element of the illuminated water column independently by measuring the frequency of the heterodyne signal. Since the frequency is directly related to the water temperature and since time is directly proportional to depth, the resultant time-temperature pattern is said to be equivalent to a temperature test profile. Movement of a submarine through the water creates internal waves that affect this profile such that the above-noted technique is said to provide a long-persisting indication of movement of a submerged vessel.
It will be noted that the laser is a sub-sea laser which is to be mounted on a submarine that executes a search pattern in various sub-sea locations. This is inherently a slow process because of the relatively low top speed of a subsurface vessel. Secondly, the technique described in the above-mentioned patent detects a long-persisting temperature change and does not effectively locate the position of the submarine that is detected. Temperature changes persist over long periods of time such that the temperature changes engendered by the passage of a submarine can exist, for instance, over 20 miles, hardly a technique suitable for localization and tracking of a submarine. Thirdly, the system operates by detecting frequency shifts which are correlated to temperature changes. Thus, while the above technique may be able to detect wake, its ability to localize and track a submarine is questionable.